The following is a tabulation of some art that appears relevant to the present disclosure:
U.S. Pats.Pat. No.Kind CodeIssue DatePatentee7,579,753B2Aug. 25, 2009Fazzio et al.7,538,477B2May 26, 2009Fazzio et al.
U.S. Pat. Application PublicationsPublication No.Kind CodePub DateApplicant2010/0117485A1May 13, 2010Martin et al.
Piezoelectric micromachined ultrasonic transducers (PMUTs) are ultrasonic transducers used to transmit and receive ultrasound. The optimal design of a PMUT includes electrical, mechanical, and manufacturing considerations. Considering electrical design, there is a need to design an appropriate electrode structure that reduces the PMUTs sensitivity to noise arising from electromagnetic interference and allows the PMUT to generate a large acoustic output signal. Considering mechanical design, a PMUT transmits and receives ultrasound in a frequency band centered at its flexural resonance frequency. Variations in the resonance frequency due to residual stress, packaging stress, and thermal stress should be minimized. Considering manufacturing, it is desirable to minimize the number of electrical connections to the PMUT in order to reduce manufacturing cost and package size, particularly when arrays of PMUTs are to be fabricated on a common substrate.
US Patent Application 20100117485 (Martin et al.) and U.S. Pat. Nos. 7,579,753 and 7,538,477 (Fazzio et al.) disclose various electrode designs for piezoelectric transducers. The common concept behind these designs is illustrated in FIG. 1, which shows a circuit 100 consisting of two piezoelectric transducers 101 and 102 connected to a differential amplifier 101. As disclosed by Martin et al, two load resistors, RL, 103 and 104 terminate the inputs of amplifier 101 to ground. Circuit 100 is intended to increase the signal to noise ratio by rejecting common mode noise signals appearing on the inputs of differential amplifier 101 by matching the impedances on the amplifier's inputs. This design is an improvement over a single-ended design wherein the electrical impedances on the two inputs of amplifier 101 are not matched. However, the prior art circuit 100 suffers from several flaws. First, if two transducers 101 and 102 are required, the cost, complexity and size are doubled. Second, the use of resistors 103 and 104 results in additional thermal noise at the input of amplifier 105, which degrades the signal-to-thermal noise ratio.
In an alternative embodiment described by Martin et al, the two piezoelectric transducers 101 and 102 are located on a single flexural membrane, as shown in top view in FIG. 2A and in cross-section view in FIG. 2B. This flexural piezoelectric transducer consists of a single piezoelectric layer 201 deposited onto a substrate 200. Electrodes 1a, 1b and 2a, 2b of piezoelectric transducers 101 and 102 are patterned into a top metal electrode layer 202 and a bottom metal electrode layer 203. While this second embodiment requires only a single flexural transducer, it suffers from a number of flaws. First, a transducer composed entirely of a piezoelectric film suffers from poor manufacturability due to residual stress variations in the piezoelectric layer. A more manufacturable design is needed that accommodates stress variations in the piezoelectric layer. Second, the transducer design shown in FIG. 2A and FIG. 2B requires four electrical connections, adding cost and complexity. A transducer design is needed that achieves good common mode noise rejection with only two electrical connections per transducer.
Finally, Martin et al. focus exclusively on the electrical performance of the transducer as a receiver (e.g. a microphone). Because a PMUT is used both to receive and transmit ultrasound, an electrode and circuit design is needed to address the performance of the PMUT as both a transmitter and a receiver.
It is within this context that aspects of the present disclosure arise.